This invention relates to the coating of elongated strip articles, such as metal sheet or strip. More particularly, the invention relates to such coating achieved by the utilization of direct coating apparatus having floating extrusion heads.
The direct coating of strip articles with layers of coating materials utilizing one-sided and two-sided direct coaters is known, for example, from U.S. Pat. No. 4,675,230 to Robert A. Innes, issued Jun. 23, 1987 and assigned to the same assignee as the present application; and U.S. Pat. No. 5,807,434 also issued to Robert A. Innes, issued Sep. 15, 1998, and also assigned to the same assignee as the present application. These patents disclose the concept of floating coating heads (often referred to as xe2x80x9ccoating diesxe2x80x9d) for single and double sided strip article coating. The disclosed coating heads each have a coating face directed towards the surface to be coated, and an elongated slot extending across the width of the strip article at right angles to the direction of advance of the strip article through the coating apparatus. The slot allows coating material to be extruded into the gap between the coating head and the adjacent surface of the strip article. On the downstream side of the slot of each coating head, an extended, generally flat, sloping surface (usually referred to as a xe2x80x9clandxe2x80x9d) is provided. This land slopes with a predetermined angle inwardly towards the surface to be coated in the direction of advance of the strip article. The space into which the coating material is extruded consequently narrows in the direction of strip advance, and this causes the coating material to be compressed in the gap and consequently to exert an outward force on the land as the material is squeezed and metered to the desired coating film thickness. At least one of the coating heads is movable generally at right angles to the strip article and is urged by some form of pushing arrangement towards the strip. The force generated by the coating material on the land balances the force provided by the pushing arrangement pressing the coating head towards the strip article, causing the coating head to float on the newly forming film of coating material without actually touching the strip article itself. This floating effect allows a constant thickness of coating material to be applied to the strip surfaces regardless of any non-planar condition of those surfaces, since the floating coating heads follow any contours or irregularities of the strip thickness as the strip article is advanced through the coater apparatus.
Direct coaters of this kind can be used for applying various kinds of solvent-borne coatings (e.g. paints, lacquers, enamels, etc.). For example, U.S. Pat. No. 5,622,562 to Innes et al., which issued on Apr. 22, 1997 and also is assigned to the same assignee as the present application, describes a similar coating apparatus and method for coating strip articles with layers of molten polymer material.
In direct coaters of this kind, the coating dies are usually quite wide so that they may extend completely across the width of a sheet article to be coated. Die lengths of 30 inches (76.2 cm) or more are usual. When thin coatings are to be applied, e.g. coatings of 10 microns (xcexcm) or less, the profile of the coating face of each coating die must closely follow the profile of the adjacent part of the surface to be coated (viewed in the direction transverse to the direction of advance of the sheet article). If this is not the case, the thickness of the coating applied to the surface will vary to an extent that will be noticeable in the finished coated article. This can be a problem, particularly when direct coaters are used for applying molten polymer materials because the coating heads have to be heated to provide proper fluidity for the coating material, and temperature differentials created within the coating heads may cause distortion and a lack of uniformity of the die profile.
For example, two polymer coating materials commonly employed for this type of coating are polypropylene and polyethylene, and these require coating temperatures in the range of 200-300xc2x0 C. When using coating temperatures in this range, temperature differentials within the coating head in the range of 50-100xc2x0 C. are usual, and this may produce considerable distortion of the die over its width. Generally, the coating heads are supported by unheated metal support plates at the rear surface of the coating heads opposite to the coating faces. The rear portions of the coating heads are therefore usually cooler than the coating face, so the coating face becomes xe2x80x9ccrownedxe2x80x9d over the width of the coating head, i.e. bent in a convex bow so that the center of the coating face approaches the surface to be coated more closely than the ends of the coating face. For a 30 inch. (76.2 cm) die, the distortion from the desired flat profile may amount to 250 xcexcm (0.01 inch). obviously, this is completely unacceptable for coatings intended to be a uniform 10 xcexcm in thickness.
Calculations based on certain coating head dimensions (40 inches longxc3x977 inches deep) and a temperature differential (top to bottom) in the range of 50xc2x0 C. predict a deflection at the center line of the die relative to the ends of about 0.017 inches (about 425 xcexcm)(assuming that the expansion co-efficient of steel is about 1.2xc3x9710xe2x88x925/xc2x0 C.). For a coating head bolted to a steel plate, the bending effect will be partially resisted by the plate, and the observed value of about 0.010 inch (about 250xcexcm) is therefore consistent with the predicted curvatures.
Since the coating heads are pushed towards the surface to be coated by a pushing force, it is possible to modify the pushing force over the length of each die to compensate for the distortion produced by temperature differentials. However, to compensate for the observed distortions, relatively high loads are required and this necessitates a strong supporting frame, thus complicating the coating apparatus and requiring increased construction costs. Also, the totals of such loads may be inconsistent with that required to obtain a desired film thickness. Indeed, it is possible to encounter distortions beyond the range of those correctable by adjustment of the load profile. This is particularly frequent in the case of two-sided direct coaters where the loading device on one die must correct for the sum of the distortions in both opposed dies.
Accordingly, there is a need for a process and apparatus for adjusting for distortions of coating heads in direct coater apparatus, particularly when those coaters are used for coating with materials that require elevated coating temperatures.
An object of the present invention is to provide a process and apparatus for coating a sheet article with a coating film of uniform thickness across the width of the sheet article.
Another object of the present invention is to compensate for distortions of the coating profiles of coater heads used in direct coating apparatus.
Another object of the invention is to prevent irregularities in thin coatings of coating materials that are applied in heated molten form by direct coater apparatus.
Thus, according to one aspect of the invention, there is provided, in a coating apparatus including at least one floating coating head having a coating profile that becomes distorted by heat during use, the improvement which comprises providing heaters in or adjacent to specific regions of said at least one coating head to equalize temperatures throughout said coating head t substantially prevent or correct distortion of said coating profile.
According to another aspect of the invention, in a coating apparatus for two-sided coating of an elongated strip article, having coating heads in register on opposite sides of the strip article for applying a coating of material, each coating head including a coating surface having a coating profile, and at least one of said heads floating on a layer of applied coating material under a pressing force directed towards the strip article, wherein one of said coating heads is provided with means for varying heat flux to or from said coating head to modify the coating profile of said one coating head, thus enabling the coating profile of the one head to conform to the coating profile of the coating head on an opposite side of the strip article.
More specifically, according to another aspect of the invention, there is provided a process of coating a strip article with a film of coating material by passing the strip article through a coating apparatus having at least one floating coating head provided with a coating face, extending from one end of the coating head to an opposite end, facing and extending laterally across a surface of the strip article to be coated, said coating face including an elongated slot and an extended surface forming a land positioned downstream of the slot and arranged at an effective land angle with respect to the surface to be coated causing said land to slope inwardly towards said surface in a direction of movement of said strip article through said apparatus, and extruding a liquid coating material from the slot onto a surface of the strip article to be coated to be engaged by the land to meter said coating material to a desired coating thickness and to create a force on said at least one coating head that balances a pressing force urging the coating head towards the surface to be coated, wherein said coating face of said at least one coating head has a preferred coating profile between said one and said opposite ends of the coating head, and wherein distortions of said preferred coating profile during coating of said strip article are substantially prevented or corrected by modifying heat flux to or from selected parts of said coating head.
According to another aspect of the invention, there is provided a coating apparatus for coating a strip article with a film of coating material, comprising: a strip article feed for advancing a strip article through said apparatus along a strip article path; at least one floating coating head provided with a coating face, extending from one end of the coating head to an opposite end, facing and extending laterally across a surface of the strip article to be coated, said coating face including an elongated slot and an extended surface forming a land positioned downstream of the slot and arranged at an effective land angle with respect to the surface to be coated causing said land to slope inwardly towards said surface in a direction of movement of said strip article through said apparatus; a material feed for supplying liquid coating material to said coating head; and force applying means for urging said coating head towards said strip article, whereby said strip article advanced through the apparatus by the strip article feed is coated with coating material exuded from said elongated slot of the coating head and is engaged by said land to meter said coating material to a metered coating thickness, and force generated on said land by said coating material is balanced by opposing force from said force applying means causing said coating head to float on said coating material; wherein said coating face of said at least one coating head has a preferred coating profile formed between said one and said opposite ends of the coating head, and wherein said at least one coating head is provided with one or more heat flux modifying devices for modifying heat flux to or from selected parts of said coating head effective to prevent or correct substantial distortions of said preferred coating profile during coating of said strip article.
According to yet another aspect of the invention, there is provided a coating head for a direct coating apparatus employing at least one floating coating head for coating a strip article with a film of coating material, said coating head comprising: a coating face, extending from one end of the coating head to an opposite end, said coating face including an elongated slot and an extended surface forming an angled land positioned adjacent to the slot; and an inlet for coating material to the coating head for delivery to the slot; wherein said coating face has a preferred coating profile formed between said one and said opposite ends of the coating head; and wherein said coating head is provided with one or more heat flux modifying devices for modifying heat flux to or from selected parts of said coating head effective to prevent or correct substantial distortions of said preferred coating profile during coating of said strip article.
The term xe2x80x9ccoating profilexe2x80x9d in the context of this invention means the profile of the coating surface that affects the ultimate thickness of the metered coating layer. This is usually the profile of the downstream edge of the coating land in elevational view as the strip advances towards the viewer.
The term xe2x80x9cmodifyingxe2x80x9d the heat flux may mean supplying heat to the coating head of the coater apparatus, or reducing heat flow from the coater head.
The distortions from the preferred profile are generally substantially prevented by providing heat to specific regions of the coating head to equalize temperatures throughout the coating head.
The heat flux is generally provided to a rear wall of the coating head opposite to the coating face, more heat being provided close to the lateral ends of the coating head than to a region of the rear wall located centrally between its lateral ends.
The coating head may be supported by a backing support plate, and distortions may be substantially prevented by providing heat to specific regions of the backing support plate to equalize temperature throughout the coating head. The backing support plate has lateral ends adjacent to lateral ends of said coating head, and more heat is usually provided to regions of the backing support plate close to the lateral ends thereof than to a region of the backing support plate located centrally between the lateral ends thereof.
The heat may be provided by locating electrical heaters in specific regions of the coater head or backing plate, and operating the heaters when required to substantially prevent distortions.
If desired, distortions of the coating profile may be measured and, when the distortions exceed a predetermined amount, heat flux may be modified to reduce such distortions. The distortions may be measured directly by measuring positions of points on the coating surface, or indirectly by measuring temperature differentials at points on said coating head.